This invention relates to a method for forming silica-titania vitreous material and, more particularly, to a method for forming a clear vitreous material substantially comprising silica-titania binary which contains up to about 40 mole percent titania without melting the constituents used in forming the binary vitreous material.
Silica-titania is known as a glass forming system and constitute some of the most refractory glasses known. These glasses also exhibit a very low thermal expansion. For some applications, the silica-titania binary system possesses certain additional advantages over quartz, such as high temperature applications where some resistance to alkali vapor attack is desirable, lamp envelopes being an example. The lowest liquidus in the silica-titania binary is 1550.degree. C. which corresponds to 10 mole percent titania-90 mole percent silica. As the titania content increases, the liquidus temperature rises rapidly and is 1780.degree. C. at 20 mole percent titania. Because of the considerable difficulties encountered in the melting and the processing of these glasses, their use is quite limited. Most glasses in this system are limited to a titania content not to exceed 10 mole percent although glasses in the compositional range of 10 to 20 mole percent titania have been investigated with flame hydrolysis of the chlorides or induction melting at temperatures in excess of 1800.degree. C. which are used to produce the glasses.
It is well known to prepare particulate metal oxides by hydrolyzing the metallic alkoxides thereof in order to produce an intermediate product which is then pyrolyzed to oxide form. A method for treating aluminum alkoxides to produce active alumina is disclosed in U.S. Pat. No. 3,944,658, dated Mar. 16, 1976 to B. E. Yoldas, the present applicant. A method for producing silica glass fibers and transparent silica glass from silicon tetraethoxide by hydrolysis, polymerization and heating at less than the melting temperature of silica is disclosed in Chem. Abstracts 90:141247f.
U.S. Pat. No. 2,689,858, dated Sept. 21, 1954 to Thomas Boyd discloses polymers of tetra-organo derivatives of orthotitanic acid which are soluble in organic solvent. These polymers are prepared by reacting alkyl titanates with a small amount of water to produce a haze-free polymer. The polymers have utility as modifiers for condensation resins.
U.S. Pat. No. 3,460,956, dated Aug. 12, 1969, to Dahle discloses the formation of thin titania coatings from clear alcohol-water solutions which are modified by the addition of lactic acid or nitric acid.
U.S. Pat. No. 2,768,909, dated Oct. 20, 1956, to Haslam discloses applying tetraalkyl titanate to a substrate and then hydrolyzing same from the moisture in the atmosphere in order to form a thin film which in some cases can be transparent.
U.S. Pat. No. 3,094,436, dated June 18, 1963, to Schroder discloses depositing on a substrate partially hydrolyzed organic titanic acid and/or silicic acid esters in an alcohol vehicle, which on heat treatment convert to transparent reflection-reducing film coatings.
In Proceedings, Tenth International Congress on Glass (July 1974) Kyoto, Japan, Part II, 13-44 to 13-48, report by Kimiya et al, are disclosed titania-silica glass systems prepared by mixing titanium alkoxide and silicon alkoxide and then hydrolyzing same by exposure to the humidity in the air for a week or so to form a solid or gel. The resulting gelled or solid masses are then heat treated at temperatures ranging from 300.degree. C. to 1200.degree. C.